Thermoplastic resin films such as polyamide films and polyester films have excellent strength, transparency and moldability, and are consequently widely used as packaging materials. However, because these thermoplastic resin films also exhibit reasonably high levels of permeability to gases such as oxygen, if this type of thermoplastic resin film is used for packaging general foodstuffs, retort foods, cosmetics, medical supplies, or agricultural chemicals or the like, then during long-term storage, gases such as oxygen can permeate through the film, causing deterioration of the package contents.
As a result, laminated films produced by coating the surface of a thermoplastic resin with an emulsion or the like of polyvinylidene chloride (hereafter abbreviated as PVDC), thereby forming a PVDC layer with good gas barrier properties, are widely used for applications such as food packaging. However, PVDC generates organic substances such as acidic gases on incineration, and with recent advances in environmental awareness, there is considerable demand for replacing PVDC with other materials.
One example of an alternative material to PVDC is polyvinyl alcohol (hereafter abbreviated as PVA), which does not generate toxic gases, and exhibits excellent gas barrier properties under low humidity conditions. However, as the humidity increases, the gas barrier property declines rapidly, so that in most cases, PVA films cannot be used for wrapping foods that contain moisture.
One example of a polymer known to improve upon the deterioration in gas barrier properties seen for PVA under high humidity conditions is a copolymer of vinyl alcohol and ethylene (hereafter abbreviated as EVOH). However, in order to ensure that the gas barrier property is maintained at a practical level under high humidity, the proportion of ethylene within the copolymer must be increased to a certain level, but the resulting polymer becomes difficult to dissolve in water. Accordingly, in order to produce a coating agent using EVOH with a high ethylene ratio within the copolymer, either an organic solvent, or a mixed solvent of water and an organic solvent must be used. However the use of organic solvents is undesirable from an environmental perspective, and also results in increased costs due to the necessity of providing a process for recovering the organic solvent.
Examples of methods that have been proposed for coating a film with a liquid composition comprising a water-soluble polymer in order to achieve favorable gas barrier properties even under conditions of high humidity include methods in which an aqueous solution comprising PVA and a partially neutralized product of polyacrylic acid or polymethacrylic acid is coated onto a film, and a heat treatment is then conducted to effect cross-linking via ester linkages between the two polymers (see patent references 1 to 7). However, in the methods proposed in these references, either a high-temperature heat treatment or a heat treatment over an extended period is required to achieve favorable gas barrier properties, and because large quantities of energy are therefore required during production, the impact on the environment is not insignificant. Moreover, if a high-temperature heat treatment is employed, then not only is there an increased danger of color changes or decomposition of the PVA and the like that constitute the gas barrier layer, but deformation such as wrinkling can occur in the plastic film substrate or the like to which the gas barrier layer is laminated, meaning the product cannot be used as a packaging material. In order to prevent deterioration of the plastic substrate, a special heat-resistant film that is capable of withstanding the high-temperature heat treatment must be used as the substrate, but this creates problems of practicality and economic viability. On the other hand, if the temperature of the heat treatment is lowered, then treatment must be conducted over an extremely long period, causing a deterioration in productivity.
Furthermore, investigations are also being conducted into resolving the above problems associated with PVA film by introducing cross-linking structures into the PVA. However, although the humidity dependence of the oxygen gas barrier property of PVA film typically decreases with increasing cross-linking density, the inherent oxygen gas barrier property of the PVA film under dry conditions tends to deteriorate, meaning it is extremely difficult to achieve a favorable oxygen gas barrier property under high humidity conditions. Cross-linking of polymer molecules generally improves the water resistance, but the gas barrier property describes the ability of the material to prevent the penetration or diffusion of comparatively small molecules such as oxygen, and a favorable gas barrier property can not always be achieved simply by cross-linking the polymer. For example, three dimensional cross-linked polymers such as epoxy resins and phenolic resins do not exhibit effective gas barrier properties.
Methods have also been proposed which, although using a water-soluble polymer such as PVA, are capable of providing gas barrier laminates with favorable gas barrier properties even under high humidity, by conducting heat treatments at lower temperatures or for shorter time periods than those conventionally used (see patent references 8 to 10).
Although using water-soluble polymers, the gas barrier layer-forming coating materials disclosed in the patent references 8 to 10 are able to form gas barrier laminates with superior gas barrier properties to those conventionally obtained, by conducting heating at lower temperatures or for shorter time periods than those employed for the coating agents disclosed in the patent references 1 to 7. However, with the methods disclosed in the patent references 8 to 10, in which an esterification reaction is conducted between the hydroxyl groups of PVA and the COOH groups within an ethylene-maleic acid copolymer, or in which metal cross-linking structures are introduced, there is a limit to the degree of improvement than can be achieved in the gas barrier property under high humidity.
As a result, other methods have been proposed that improve on the above techniques in order to achieve even better gas barrier properties (see patent references 11 to 14). These references disclose that, by heat treating a gas barrier coating material comprising PVA and a composition prepared by partially neutralizing an ethylene-maleic acid copolymer with a specific metal salt, a gas barrier coating can be obtained that is superior to those disclosed in the patent references 8 to 10, and that by heat treating the thus obtained gas barrier coating in the presence of water, or in the presence of water comprising a specific metal ion, an even more superior gas barrier coating can be obtained. Examples of the method used for conducting the heat treatment in the presence of water (or water comprising a specific metal ion) include immersion in hot water, hot water spraying, storage under high humidity conditions, and steam heating, wherein the treatment temperature is preferably not less than 90° C., and the treatment time is preferably not less than 1 minute.
However, in these types of methods, because the film with the gas barrier layer coated thereon must be in contact with water for a comparatively long time, the production process can be expected to be more complex, and the productivity is expected to worsen. Moreover, the effects of heat and water absorption on the film during the treatment step are considerable, meaning that, for example, in those cases where a highly water-absorbent film such as a polyamide is used as the substrate, adverse effects on the product quality such as deformation and curling are a concern.
As described above, although there are increasing demands for further improvements in the gas barrier properties under conditions of high humidity, obtaining a high-quality gas barrier laminate with superior performance in an industrially efficient manner has proven difficult with the conventional technology. Moreover, incorporating a metal compound into the coating agent causes a deterioration in the film-forming properties, and causes a deterioration in the adhesive strength, the heat resistance and the water resistance when a laminated structure (or a laminated product) is prepared with a heat seal layer, meaning performance problems arise in practical applications.
(Patent Reference 1) Japanese Patent Laid-Open No. H06-220221
(Patent Reference 2) Japanese Patent Laid-Open No. H07-102083
(Patent Reference 3) Japanese Patent Laid-Open No. H07-205379
(Patent Reference 4) Japanese Patent Laid-Open No. H07-266441
(Patent Reference 5) Japanese Patent Laid-Open No. H08-041218
(Patent Reference 6) Japanese Patent Laid-Open No. H10-237180
(Patent Reference 7) Japanese Patent Laid-Open No. 2000-000931
(Patent Reference 8) Japanese Patent Laid-Open No. 2001-323204
(Patent Reference 9) Japanese Patent Laid-Open No. 2002-020677
(Patent Reference 10) Japanese Patent Laid-Open No. 2002-241671
(Patent Reference 11) Japanese Patent Laid-Open No. 2004-115776
(Patent Reference 12) Japanese Patent Laid-Open No. 2004-137495
(Patent Reference 13) Japanese Patent Laid-Open No. 2004-136281
(Patent Reference 14) Japanese Patent Laid-Open No. 2004-322626